Add CI workflow to check for broken links in docs

On every push or pull request, check for broken links in the Markdown files of the repository.

Author: per1234

.github/workflows/check-links.yml

@@ -0,0 +1,30 @@
+name: Check Links
+
+# See: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows
+on:
+  push:
+    paths:
+      - ".github/workflows/check-links.yml"
+      - "**.md"
+  pull_request:
+    paths:
+      - ".github/workflows/check-links.yml"
+      - "**.md"
+  schedule:
+    # Run every Tuesday at 3 AM UTC to catch breakage caused by changes to the linked sites.
+    - cron: "0 3 * * TUE"
+  workflow_dispatch:
+  repository_dispatch:
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Check links
+        uses: gaurav-nelson/github-action-markdown-link-check@v1
+        with:
+          use-quiet-mode: yes